Polypropylenes have wide applications because of excellent optical properties and mechanical properties thereof. In recent years propylene-ethylene copolymers produced with a metallocene catalyst have been developed. Because the copolymers have narrow composition distribution and do not contain low molecular weight compounds or low crystallinity components that may cause stickiness or reduction in rigidity, they are known to provide practically excellent films having low stickiness or blocking (see Patent Literature 1) and thus have been used in the field of films and sheets.
Further, olefin thermoplastic elastomers have been developed which are obtained with a metallocene catalyst and have preferable properties such as appropriate flexibility and strength, transparency, appropriate heat resistance, improvement in blocking property and being devoid of deterioration in appearance over time (see Patent Literature 2) and have been widely used not only for films and sheets but also for fibres, non-woven fabric, various containers and moulded products.
In recent years reduction in the moulding temperature has been sought in view of improvement inaccuracy of the thickness in the width direction of films, reduction in production cost and reduction in solidification time during moulding. The metallocene type copolymers have not only narrow composition distribution but also narrow molecular weight distribution and thus generally have low extrusion property. It is an industrial issue in terms of extrusion property that films discharged from dies by extrusion moulding have impractical appearances as a result of surface roughness referred to as sharkskin and melt fracture.
The surface roughness may be addressed by adjustments of moulding conditions such as an increased temperature of dies or an increased width of the outlet of dies. However, an increase in the temperature may cause problems of heat deterioration of propylene-ethylene copolymers resulting in generation of odour, an increase in the neck-in upon taking up onto rolls from dies and an increase in variation of the thickness. When the width of the outlet of dies is increased, not only the issues other than the heat deterioration are difficult to be solved but also the accuracy of the thickness in the width direction of films is decreased (see Non Patent Literature 1).
In order to improve the extrusion property of propylene-ethylene copolymers obtained with metallocene catalysts, various attempts have been made so far.
First, in order to widen the molecular weight distribution by adjusting a catalyst or a polymerization method, there have been proposed (1) to use a specific metallocene catalyst in order to widen the molecular weight distribution (Patent Literature 3 and 4), (2) to carry out multi-stage polymerization in order to obtain propylene-ethylene copolymers containing two components having different molecular weights (Patent Literature 5 to 7) and (3) to carry out graft modification or copolymerize a macromonomer or a diene to introduce long chain branches (Patent Literature 8 to 12).
With regard to the above proposal (1), Patent Literature 2 for example discloses a propylene copolymer obtained with a metallocene catalyst and having Mw/Mn of 4.0 to 7.7. However, Patent Literature 2 does not disclose Mz/Mn which indicates the presence of an ultrahigh molecular weight component contributing to an improvement in the extrusion property. Moreover, it is well known that when a propylene copolymer obtained with a Ziegler-Natta catalyst and a propylene copolymer obtained with a metallocene catalyst both of which have the same weight average molecular weight are compared for the Mz/Mn thereof, the propylene copolymer obtained with a Ziegler-Natta catalyst has an apparently higher Mz/Mn and has better extrusion property, and thus it is apparent that the above proposal is insufficient for improving the extrusion property.
With regard to the above proposal (2), in order to improve the surface roughness while maintaining the melt flowability, it is required to confer a difference in molecular weight between the low molecular weight component and the high molecular weight component. However, it is difficult to produce a high molecular weight component with a metallocene catalyst and thus the molecular weight of the high molecular weight component needs to be increased. This results in an increase in the difference in the molecular weight and is not preferable because appearance defects referred to as gel or fisheye may be produced.
With regard to the above proposal (3), it is difficult to control a side reaction of crosslinking which may induce production of gel and thus increase appearance defects.
In order to blend materials having excellent extrusion property, there have been proposed (4) to blend with another resin such as polyethylene (Patent Literature 13) and (5) to blend with a propylene-ethylene copolymer having a wide molecular weight distribution (Patent Literature 14 and 15).
With regard to the above proposal (4), polypropylene is not compatible with polyethylene, and thus when another resin is blended until an improvement in the extrusion property is obtained, transparency is significantly deteriorated, resulting in limitation in use application thereof.
With regard to the above proposal (5), Patent Literature 14 for example discloses a copolymer resin composition containing 1 to 70% by weight of a propylene random copolymer (A) obtained with a metallocene catalyst and 30 to 99% by weight of a conventional propylene random copolymer (B) obtained with a Ziegler-Natta catalyst. However, the propylene random copolymer (B) used in the invention has a MFR of as high as 9.2 and does not contain a high molecular weight component that is required in order to improve the surface roughness, and thus the invention does not provide an improvement in the surface roughness.
Meanwhile, Patent Literature 15 discloses a method for improving extrusion property by blending a polypropylene obtained with a metallocene catalyst with a polypropylene having a high melt tension obtained by two-stage polymerization using a metallocene catalyst. However, the polypropylene having high melt tension used in the invention does not include a comonomer and thus when extrusion moulding is carried out at a relatively low temperature of about 180° C., a long molecular chain having long relaxation time causes oriented crystallization while flowing in dies, resulting in solid-liquid phase separation-like situation at the outlet of the dies. Thus when a draft ratio (ratio between the cooling roll take-up speed and the resin discharge speed) is increased, the obtained moulded products may not be suitable for practical use because cracks are produced in the liquid layer.
Further, there has been a proposal (6) to use a specific additive to improve the surface roughness (Patent Literature 16). In this proposal, addition of a fluoroelastomer as a processing aid to suppress production of melt fracture is disclosed. It is generally known that addition of a fluoroelastomer provides a fluoroelastomer coating on the surface of dies during the flow of a resin therein, reduces the resin pressure in the vicinity of the wall of the dies and thus suppresses production of melt fracture as a result. However, when continuous moulding over a prolonged period of time is carried out with this manner, the fluoroelastomer coating on the wall of the dies is deteriorated due to heat, is burnt and appears as black spots in films, causing practical issues.
An invention of a biaxially stretched multi-layer film has been disclosed which exploits excellent properties of a propylene polymer obtained with a metallocene catalyst such as excellent transparency and gloss and low stickiness and blocking. Patent Literature 17 for example discloses an invention of a biaxially stretched multi-layer film including a polypropylene film and a sealant layer stacked thereon containing specific silica particles and a propylene random copolymer obtained with a specific metallocene catalyst. However, propylene resins obtained with a metallocene catalyst have inferior mouldability and thus have drawbacks in terms of film mouldability per se. When, in spite of these drawbacks, such a resin is further coextruded with a propylene resin having a different flowability to form a laminate film, the laminate film has significantly deteriorated optical properties due to sharkskin produced at the outlet of dies or appearance defects resulting from the roughness at the interface of the stacked layers produced in the dies. Further, narrow molecular weight distribution and composition distribution which are characteristics of propylene resins obtained with a metallocene catalyst cause deterioration in hot tack property expressing high speed filling ability and heat sealability. Thus the film has insufficient properties so as to be used as films for packages.
Patent Literature 18 discloses an invention of a polypropylene biaxially stretched multi-layer film obtained by laminating on a substrate layer of an olefin polymer a surface layer of a propylene-ethylene random block copolymer obtained by sequential polymerization so that the heat seal strength and the thickness of the surface layer fulfil specific formulae. Although the propylene-ethylene random block copolymer obtained with a metallocene catalyst may provide a multi-layer film having excellent heat sealability at low temperatures and blocking resistance, issues such as deterioration in mouldability and reduction in hot tack property are not solved.
In order to address low mouldability of propylene (co) polymers obtained with a metallocene catalyst, Patent Literature 19 discloses an invention of a polypropylene resin composition including a propylene homopolymer obtained with a metallocene catalyst and having a relatively low intrinsic viscosity [η] and a propylene homopolymer obtained with a metallocene catalyst and having a relatively high intrinsic viscosity [η].
However, the propylene homopolymers do not exhibit sufficient heat sealability at low temperatures. Further, the difference in the intrinsic viscosity [η] between two propylene homopolymers obtained with a metallocene catalyst is low and the intrinsic viscosity [η] of the propylene homopolymer having a relatively low intrinsic viscosity [η] and obtained with a metallocene catalyst is merely within the range from 1.0 to 2.0 dl/g, which range of intrinsic viscosity [η] cannot sufficiently improve the mouldability. Thus the laminate film obtained by coextrusion has significantly deteriorated optical properties due to sharkskin produced at the outlet of dies or appearance defects resulting from the roughness at the interface of the stacked layers produced in the dies.
Patent Literature 20 discloses an invention of a laminate film including a first layer of a polyolefin resin A having a molecular weight distribution of 3.5 or less and a second layer of a polyolefin resin B, wherein the flowability parameter represented by zero shear viscosity [η0] and the molecular weight distribution W of the resins is within a specific range. Patent Literature 20 describes that the laminate film does not generate appearance defects (scale patterns) and has excellent blocking resistance, transparency, moisture proof property and heat sealability.
However, merely adjusting the zero shear viscosity [η0] and the molecular weight distribution W of the polyolefin resin B used in the substrate layer and the polyolefin resin A used in the sealant layer could not sufficiently improve the mouldability. Particularly, the invention disclosed in Patent Literature 20 provides an improvement in properties of the polyolefin resin B used in the substrate layer and thus when the polyolefin resin A is the one obtained with a metallocene catalyst, low mouldability which is intrinsic to polyolefin resins (e.g., propylene random copolymers) obtained with a metallocene catalyst is not addressed.
Further, Patent Literature 21 discloses an invention pertaining to a propylene laminate film including 50 to 99% by weight of a high molecular weight ethylene-propylene random copolymer (A) having an ethylene content of 1.0 to 6.0% by weight and a Mw of 270000 to 460000 and polymerized using a metallocene catalyst and to 50% by weight of an ultralow molecular weight ethylene-propylene random copolymer (B) having an ethylene content of 1.0 to 6.0% by weight and a Mw of 29000 to 130000 and polymerized using a metallocene catalyst, the film including a sealant layer of a propylene resin composition having a MFR of 2.0 to 20 g/10 min.
This invention provides sufficient improvement in interface roughness, however, does not improve the hot tack property. Further, recently there is a need for an increase in static flow length in dies in order to improve accuracy of the thickness of films or an increase in a discharge amount in order to improve productivity. It appears that the invention of Patent Literature 21 which has insufficient relaxation time of the composition causes problems of surface roughness and thus may not be able to meet these requirements.